The Effects of New Design of Access Hole on Porcelain Fracture Resistance of Implant-Supported Crowns.

STATEMENT OF THE PROBLEM
One disadvantage of cement-retained crowns is the lack of predictable irretrievability. This problem can be overcome through designing a screw access hole in the metal substructure of cement-retained restoration and using porcelain stain to define this area.


PURPOSE
This study aimed to evaluate the influence of existence of screw access hole on porcelain fracture resistance of metal-ceramic implant-supported crowns.


MATERIALS AND METHOD
Thirty six standardized metal-ceramic crowns were fabricated and divided into 3 groups (n=12); group 1 conventional cement-retained metal-ceramic crowns as control group, group 2 cement-retained MC crowns in which porcelain stain was used to define the location of screw access channel, and group 3 cement-retained metal-ceramic crowns in the metal substructure of which a hole and ledge was designed in the location of screw access channel. The specimens were cemented (TempBond, Kerr) to their dedicated abutments. A hole was made in the location of screw access channel in group 2 and 3 and filled with photo-polymerized composite resin (3M; ESPE). All specimens were thermocycled and loaded in universal testing machine at crosshead speed of 2mm/min until fracture. Mean values of load at fracture were calculated in each group and compared with One-way ANOVA (α=0.05).


RESULTS
Mean value of the load required to fracture the restorations was 1947±487 N in group 1, 1927±539 N in group 2, and 2170±738 N in group 3. No statistically significant difference was found between the fracture resistance values of the three groups (p> 0.05).


CONCLUSION
Presence of screw access channel in cement-retained implant restorations does not compromise fracture resistance.


Introduction
Dental implants have been approved for their durable predictable success in treatment of both completely [1][2] and partially edentulous patients. [3][4] Metal ceramic restorations are generally used during the restoration phase of implant, particularly in treatment of partially edentulous cases. [5] Implant-supported crowns can be either screw-retained [6][7] or cement-retained; [8][9] however, there still exists controversies over the best retention type for implant-supported restorations. [10][11][12][13][14] Retrievability is the main advantage of screwretained prosthesis, [15][16] because the prosthesis might need to be removed to repair the crown (in case of ceramic-fracture or screw loosening). This feature also provides a better assessment of oral hygiene and peri-implant probing, as well as replacing the components in case of screw loosening or fracture. [16][17][18] However, the laboratory procedures required for screwretained restorations are usually more sophisticated, expensive, and associated with inherent mechanical complications such as screw-loosening and fractures. [19][20] It is generally difficult and costly to remove and replace the fractured screws. [15] Furthermore, natural occlusal morphology might be interfered due to the presence of a screw access opening, [21] which might also disrupt the porcelain continuity and lead to unstable occlusal contents. [22][23] As reported by a number of studies, presence of screw access opening in these restorations reduces the fracture resistance of the porcelain. [15,22,[24][25] Among the advantages of cement-retained restorations are the lower costs of fabrication, facilitated procedure of implant restoratives, and a better passive fit, as well as preventing the interference of screw access opening with the esthetic or the occlusion of the restoration. [8,25] Meanwhile, cement-retained restorations have some drawbacks including the difficulty in retrieving and removing the excess cement around the crown, in addition to cement loss which may lead to periimplant inflammation. [15,[26][27] Cement-retained restorations have been advised for treating partial edentulous patients with implant [16] and they are better to be the first treatment option when esthetic is concerned. [28] Besides, cement-retained implant prosthesis is chiefly concerned with the difficulty of being removed when the abutment screw has loosened or the porcelain has fractured and need to be corrected.
Literatures have suggested several methods to provide retrievability of cement-retained implant restorations; using provisional cement is one of them, although the retention may be damaged. [28] Placing a lingual retrieval slot at the abutment/prosthesis interface is another solution. [29] Other approaches are making use of ceramic stain on the occlusal surface of posterior restoration, [15] digital photographs or vacuum-formed templates to identify the position of screw. [30] In these methods, the screw channel is filled with composite resin when the restoration is removed. One concern is that, existence of screw channel in the framework may jeopardize the strength of restoration.
In the present study, a special feature was designed in the metal framework to support the remaining porcelain after the screw access channel was created.
There is little data in the literature regarding the fracture resistance of cemented prostheses with screw access channel. The current investigation aimed to evaluate the influence of access hole in the occlusal surface of cement-retained implant restoration with and without specially designed feature on the fracture resistance of the restoration. The null hypothesis was that creating screw access on cement-retained crowns to permit retrievability would not compromise the strength of restoration.

Discussion
With respect to the obtained results of the present study, the null hypothesis cannot be rejected. The fracture resistance of specially designed cement-retained crowns with screw access was higher than the other two groups; this difference was not significant though (p> 0.05).
Zarone et al. [23] evaluated the fracture resistance of screw-retained versus cement-retained single metalceramic crowns. They found no significant difference between the two groups. However, fracture resistance values of cement-retained crowns were higher; which is in accordance with the results of the current study.
In another study, Karl et al. [32] observed more chipping fractures in screw-retained fixed dental prostheses than cemented models. They did not fill the access opening with any materials. The authors proposed filling the access hole with intraoral ceramic-repair resin composites to stabilize the occlusal surface of screwretained implant restorations.
Several studies [22,[32][33][34] revealed the fracture resistance of screw-retained restorations with screw access hole to be less than cement retained restorations. There were some limitations in the present study, one of which was the single static force that was used to load the specimens and it differed from the dynamic load in the oral environment. In mouth, the restoration may also fracture due to fatigue loading. [37] In the present study, the crowns were cemented using zinc oxide-eugenol cement which is used more clinically and no permanent cement was tested.
Future researchers are recommended to investigate larger sample sizes under physiologic fatigue loading. Employing different types of cements for luting the specimens is also advised.

Conclusion
Within the limitations of this study, the following con-